Here is a fairly straight-forward acoustic guitar modification project you can do with just a few tools. Bring the sound of spring reverb directly to your instrument and better yet, use no batteries or cables to get a bigger, brighter overall sound to your performance. All example sounds are as recorded, no treatments or effects were added.
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- Power drill and bits
- Needle Nose pliers
- Wire cutters
- Dremel and drill bit (optional)
- Eyelet screws (2)
- Wooden plug with dowel
I am selling a kit with a preassembled plug and the spring I used for this project on Reverb.com. If you purchased the kit you can skip step 5, "prepare the plug". Using the spring from my kit will also spare you the process of playing Goldilocks until you find just the right size/tension and sound.
Step 1: Selecting a Guitar
Choose to do the mod on a full size acoustic guitar, as opposed to one that is 3/4 or kids-size, since you will need as much access as possible to the inner body using your hand and tools. Look for one that appears to have good structural integrity, surface blemishes are fine. I found an older nylon string guitar on a local Craigslist ad for $50.
Important, be sure you understand this project involves permanent modifications to the guitar, although only a single drill hole is necessary to the body and will appear as an ordinary strap peg upon finished. I am not personally responsible for any damages done to instruments as a result of this DIY project.
Loosen or remove the strings and finally check that you can put your entire hand inside the sound hole and still be able to move it around a bit. If this is not possible I would recommend finding someone with slightly smaller hands to help!
Step 2: Locate the Drill Hole
The guitar I used did not previously have a strap peg installed so a new hole needed to be made. If your guitar has a peg already, you can simply unscrew it, use it as a pilot hole for the new, bigger hole. You can also skip the next step.
You need to locate the middle point at width and hight of the guitar body. Luckily my guitar had a light joinery line so I just needed to measure it's height and divide that number by 2 to find it.
Use a fine tip marker at the spot you measured and you are ready to make some dust fly
Step 3: Drill a Pilot Hole
I used a Dremel tool with drill bit rather than a power drill for the pilot hole since it is easier to control to avoid the drill wandering and ruining the finish. This is totally optional and you could definitely do this all with the same power drill. I set the Dremel to a fairly low speed and lowered it slowly holding with two hands. Always use eye protection when using a Dremel or power drill.
Step 4: Drill the Actual Hole
Choose a drill bit that is slightly larger than the size of your smaller eyelet that will attach to the wooden plug. If you are using the parts from my kit, the drill bit size will be 5/16" or 7.94 mm.
Gently pump the drill, adding pressure to remove the dust as you drill. My guitar had a fairly thick piece of bracing (as seen in the photo) that took longer than expected for me to drill all the way through. For comparison of size, I put a guitar string winder tool inside for the photo.
Step 5: Prepare the Plug
At this point you need to install the small eyelet screw to the plug. I used size 216 x 11/16, the diameter just slightly smaller than the size of my 5/16 drill bit. With the Dremel again I made a small pilot hole first in the plug. Ask a second person hold the plug with pliers while you do this as a safety precaution.
If you are not drilling a pilot hole in the plug, hold it firmly at the big end and try to keep it straight while twisting it with your fingers or pliers to drive it in. Now take a pair of wire cutters and clip a section of the eyelet out so that it resembles a hook. It hook must be open enough for the spring end to slip over, so test it. Also test that it fits into the hole you drilled and you know which way the open side is oriented.
Step 6: Install Second Hook Screw
You will need a slightly bigger eyelet screw than the one you used for the plug to ensure the best grip as you twist the screw. Use the cutters again to cut a section out so that it resembles a hook.
Locate the bracing closest to the back of the sound hole. Reach into the guitar and press the screw firmly into the soft wood, so as to start it. Manoever your fingers as you continue to turn it, taking care not to let the screw fall out. Depending on your angle, it may not be totally perpendicular to the bracing, but should be as tight as possible.
Orient the open side of the hook towards the sound hole opening to make it easier to attach the spring.
Step 7: Attach Spring to Plug Inside Guitar
While keeping the plug in place with one hand to ensure the hook does not move around, place your other hand into the sound hole while holding the spring. You need to operate completely by feel as you try to find the open side of the hook with the spring end furthest from you and attach it. Leave your hand from the spring now and the other side should be visible as you look into the sound hole.
Step 8: Attach Other Side of Spring
I recommend using needle nose pliers now to grab the other side of the spring, pulling to stretch it the required length to slip it over the second hook near the sound hole. I would again hold the plug against the guitar so that the other side doesn't move around while you are doing this. You shouldn't have to pull the spring with very much tension to get it to slip over the hook and attach. If you are having trouble, try looping a piece of twine through the end of the spring to help stretch it using your left hand to pull while you work with the pliers in your right. Once you have done this step you can let go and the spring will now be fixed inside the guitar!
Step 9: Test the Spring Reverb
Wind up your strings again, tune your guitar and get ready to enjoy the cavernous sounds coming from with your guitar. There should not be any rattle of the spring against the body or bracing while you are playing so you may need to adjust the string position slightly. The effect can be heard across the entire range of the guitar, but may be more noticeable in the lower strings.
Make some recordings and compare results with and without the spring. Take recordings before the spring is in so you don't have to go back to the beginning again! If you are happy with the sound I'd recommend leaving the mod in permanently to avoid any added stresses on the bracings within the guitar.